Among the various Humble Bundles, Steam sales, purchases and gifts, anyone will have happened to have a Steam code in their hands and not know how to redeem it on the famous Client.
In this simple guide, aided by the screenshots, we're going to see how to redeem a code on Steam.
Add a game to your Steam library:
From the Steam Client select "Games" then click on "Activate a product on Steam ..."
A new window will allow us to enter the code (including dashes).
Going forward we can then proceed with the download installation or simply finish.
Redeem a Steam wallet code:
From the Steam Client select "Games" then click on "Redeem a code for the Steam wallet ..." (see first screenshot).
The new window will allow us to enter the code and then associate the balance to our account.
Very simple so you just have to try it yourself!